Vascular endothelial-mediated disorders including sickle cell diseases, tissue ischemia, and circulatory disorders represent a major medical dilemma and treatment of these disorders represents an unmet clinical need. Additionally, other major vascular disorders with unmet clinical needs include pulmonary hypertension (PH) and a form of PH called pulmonary arterial hypertension (PAH). PH is a disorder characterized by abnormally high blood pressure in the lungs. Further, the narrowing of vasculature that occurs in many of these diseases causes the buildup of pressure and the heart must work harder in order to force blood through the pulmonary arteries. PH is an increase in blood pressure in the pulmonary artery, pulmonary vein, and/or pulmonary capillaries. PH is a very serious condition, potentially leading to shortness of breath, dizziness, fainting, decreased exercise tolerance, heart failure, pulmonary edema, and death. PH is generally characterized by a mean pulmonary artery pressure exceeding 25 mm Hg (3300 Pa) at rest or 30 mm Hg (4000 Pa) with exercise and the World Health Organization (WHO) has subdivided PH into five different groups:
WHO Group I—Pulmonary arterial hypertension (PAH)                Idiopathic (IPAH)        Familial (FPAH)        Associated with other diseases (APAH): collagen vascular disease (e.g. scleroderma), congenital shunts between the systemic and pulmonary circulation, portal hypertension, HIV infection, drugs, toxins, or other diseases or disorders        Associated with venous or capillary disease        
WHO Group II—Pulmonary hypertension associated with left heart disease                Atrial or ventricular disease        Valvular disease (e.g. mitral stenosis)        
WHO Group III—Pulmonary hypertension associated with lung diseases and/or hypoxemia                Chronic obstructive pulmonary disease (COPD), interstitial lung disease (ILD)        Sleep-disordered breathing, alveolar hypoventilation        Chronic exposure to high altitude        Developmental lung abnormalities        
WHO Group IV—Pulmonary hypertension due to chronic thrombotic and/or embolic disease                Pulmonary embolism in the proximal or distal pulmonary arteries        Embolization of other matter, such as tumor cells or parasites        
WHO Group V—Miscellaneous PH involves the vasoconstriction or tightening of blood vessels connected to and within the lungs. This tightening makes it harder for the heart to pump blood through the lungs, much as it is harder to make water flow through a narrower pipe as opposed to a wider pipe. Over time, the blood vessels affected by this vasoconstriction become both stiffer and thicker, in a process known as fibrosis. This fibrosis further increases the blood pressure within the lungs and impairs blood flow. In addition, the increased workload of the heart causes thickening and enlargement of the right ventricle, making the heart less able to pump blood through the lungs, causing right heart failure. As blood flowing through the lungs decreases, the left side of the heart receives less blood and this blood may also carry less oxygen than normal. Therefore it becomes harder and harder for the left side of the heart to pump to supply sufficient oxygen to the rest of the body, especially during physical activity.
In pulmonary venous hypertension (WHO Group II) there is not necessarily any obstruction to blood flow in the lungs. Instead, the left heart fails to pump blood efficiently out of the heart into the body, leading to pooling of blood in veins leading from the lungs to the left heart (congestive heart failure). This causes pulmonary edema and pleural effusions. The fluid build-up and damage to the lungs may also lead to hypoxia and consequent vasoconstriction of the pulmonary arteries, so that the pathology may come to resemble that of Group I or III.
In hypoxic pulmonary hypertension (WHO Group III), the low levels of oxygen may cause vasoconstriction or tightening of pulmonary arteries. This leads to a similar pathophysiology as PAH.
In chronic thromboembolic pulmonary hypertension (WHO Group IV), the blood vessels are blocked or narrowed with blood clots. Again, this leads to a similar pathophysiology as pulmonary arterial hypertension.
The pathogenesis of PH involves a complex and multifactorial process. Endothelial dysfunction seems to play an integral role in mediating the structural changes in the pulmonary vasculature that occur as a result of PH. These dysfunctions include disordered endothelial cell proliferation and concurrent neoangiogenesis. When this angiogenesis is exuberant, it results in the formation of glomeruloid structures known as plexiform lesions. In addition, a decrease in the vasodilators nitric oxide (NO) and prostacyclin, along with an increase in vasoconstrictors such as endothelin-1 (ET-1), serotonin, and thromboxane, have been observed in patients with PH. Because most of these mediators affect the growth of the smooth muscle cells, an alteration in their production or expression may facilitate the development of pulmonary vascular hypertrophy and the structural remodeling of the vasculature that is characteristic of PH. It is conceivable that the beneficial effects of many of the treatments currently available for PH, such as the use of prostacyclin, NO, and ET-1 antagonists, result at least in part from restoring the balance between these mediators.
In addition to the potential consequences of an imbalance in the endothelial production of various mediators, injury to the endothelium caused by PH may expose the underlying vascular tissue to diverse blood-borne factors that may further promote pathological changes. Endothelial dysfunction may also have adverse consequences on pulmonary vascular hemostasis by altering the production of anticoagulant factors. Recent reports of genetic mutations in the endothelial cells of patients with PH further underscore the role of these cells in the disease pathogenesis.
Available evidence suggests that NO is at least partially responsible for resting pulmonary vasorelaxation. Endothelial NO synthetase (eNOS) catalyzes the conversion of L-arginine to citrulline, producing NO. In addition, NO activates guanylate cyclase and increases cyclic guanine mono phosphate (cGMP) levels in smooth muscle cells, causing vasodilatation. The specific role of eNOS in pulmonary vascular tone regulation is best demonstrated in animal models. Overproduction of eNOS in transgenic mice prevents hypoxia-induced PH (3, 4).
Tissue factor (TF) is a transmembrane glycoprotein that initiates the coagulation cascade when complexed with factor VIIa and may also participate in angiogenesis. In situ thrombosis occurs in severe PH, and there are several reports linking platelet activation to the etiology of severe disease. TF has also been shown to regulate intimal hyperplasia in response to systemic arterial injury. The TF and factor VIIa complex (TF/VIIa) might play a key role in the disordered angiogenesis and intimal hyperplasia seen in PH. Lung sections from PH lung immunostained with an antibody to TF showed that alveolar epithelium and bronchi stained abundantly for TF. TF was not seen in normal pulmonary arterial vascular cells. In contrast, animals with PH had modest TF staining in diseased vessels and more pronounced TF staining in the plexiform-like lesions. The disordered angiogenesis and neointimal lesions of severe human disease might be mediated via the TF/VIIa pathway and this approach may be a better model for the vascular pathology of moderate to severe human PH. Over-expression of Tissue Factor Pathway Inhibitor (TFPI) in pulmonary vascular beds results in improved hemodynamic performance and reduced pulmonary vascular remodeling in a murine model of hypoxia-induced PH. This improvement is in part due to autocrine and paracrine effects of TFPI overexpression.
For WHO Group II pulmonary hypertension, the first approach is to optimize left ventricular function by the use of diuretics, beta blockers, Angiotension Converting Enzyme (ACE) inhibitors, etc., or to repair/replace the mitral valve or aortic valve. Where there is PAH, treatment is more challenging, and may include lifestyle changes. Treatment of PAH with digoxin, diuretics, oral anticoagulants, and oxygen therapy are conventional, but not highly effective. Newer drugs targeting the pulmonary arteries include endothelin receptor antagonists (e.g., bosentan, sitaxentan, ambrisentan), phosphodiesterase type 5 inhibitors (e.g., sildenafil, tadalafil), prostacyclin derivatives (e.g., epoprostenol, treprostenil, iloprost, beroprost), and soluble guanylate cyclase (sGC) activators (e.g., cinaciguat and riociguat). One surgical approach to PAH treatment is atrial septostomy to create a communication between the right and left atria, thereby relieving pressure on the right side of the heart, but at the cost of lower oxygen levels in blood (hypoxia). Other surgical approaches include lung transplantation and pulmonary thromboendarterectomy (PTE) to remove large clots along with the lining of the pulmonary artery.
There is thus an unmet need for improved treatments for PH, particularly PAH, for cardiac insufficiency due to partial or complete blockage of coronary arteries and/or damage due to myocardial infarction (e.g., acute or congestive heart failure and acute myocardial infarction). There is moreover a need for a means of delivering factor VIIa inhibitors on a sustained basis to treat such conditions.